1. Field of the Invention
The present invention relates to photoelectric conversion devices using single crystal silicon or polycrystalline silicon or to a method for manufacturing the photoelectric conversion devices.
2. Description of the Related Art
Global warming is an important issue which requires international efforts. Green house gases such as carbon dioxide, which are the major cause of global warming, are emitted from energy such as oil, coal, and natural gas. These energy are essential for industrial society; accordingly, under present circumstances, it is not possible to simply reduce the amount of energy used. Therefore, as an energy source for the next generation, solar photovolatics, which are environmentally friendly and emit less carbon dioxide, has attracted attention and has been widespread.
Solar photovolatics may utilize solar heat, but mainly employs photoelectric conversion devices (also referred to as solar batteries or photovoltaic devices), which convert light energy to electrical energy by utilizing semiconductor characteristics.
Photoelectric conversion devices typified by solar batteries have been already available in the market and the production thereof has been expanding year by year, supported by government measures for solar cells around the world. For example, the production of solar cells around the world in 2006 was 2521 MW and is increasing by more than 40% per annum. Photoelectric conversion devices including crystalline semiconductor have become popular worldwide, and a large part of the production is occupied by the devices including single crystal silicon substrates or polycrystalline silicon substrates.
With the increase in production of photoelectric conversion devices year by year, shortage of supply and rise in price of silicon, which is a raw material, have become serious problems in industry. In the supply-demand balance of silicon, which had been excess in supply reflecting semiconductor recession, silicon has been short of supply since around fiscal 2005 due to drastic expansion of the solar battery market in addition to the recovery of semiconductor (LSI) industry. Major silicon suppliers in the world have already tried to increase their capability of silicon production, but the increase in demand outweighs the capability. Accordingly, the shortage of supply seems to continue for some time. If such a state continues, spread of solar batteries may be seriously suppressed.
Silicon photoelectric conversion devices are classified into a bulk type, a thin film type, a single crystal type, a polycrystalline type, and the like depending on the crystal states or device structures. Bulk silicon photoelectric conversion devices occupy a large part of the present production because they can achieve a sufficient photoelectric conversion efficiency. In a typical structure of a bulk silicon photoelectric conversion device, a single crystal silicon substrate or a polycrystalline silicon substrate is provided with an n-type or a p-type diffusion layer. In order to absorb sunlight, the silicon photoelectric conversion device only needs to have a photoelectric conversion layer with a thickness of about 10 μn; however, a single crystal silicon substrate or a polycrystalline silicon substrate has a thickness several tens times or more the thickness needed as the photoelectric conversion layer. Therefore, it is difficult to say that silicon is efficiently used as a raw material. To put it in extreme terms, in a bulk silicon photoelectric conversion device, the most part of the single crystal silicon substrate or the polycrystalline silicon substrate serves as a structural body which keeps the shape of the photoelectric conversion device.
In a thin film silicon photoelectric conversion device, a silicon layer provided over a supporting substrate serves as a photoelectric conversion layer. Since a silicon layer is employed only in a region which conducts photoelectric conversion, the consumption of silicon can be drastically reduced compared with a bulk silicon photoelectric conversion device. In addition, if a glass substrate or the like which can have a large area and is inexpensive can be used as a supporting substrate, it is possible to approach a cost problem which is one of the problems which disturb the spread of solar batteries.
In a conventional thin film silicon photoelectric conversion device, since a silicon layer is formed over a supporting substrate by various kinds of physical or chemical growth methods, single crystal silicon cannot be formed; accordingly, a non-single-crystal silicon layer such as an amorphous silicon layer, a microcrystalline silicon layer, or a polycrystalline silicon layer is used. Non-single-crystal silicon has lower photoelectric conversion characteristics than single crystal silicon; therefore, thin film silicon photoelectric conversion devices have not achieved a sufficient photoelectric conversion efficiency. Therefore, the use of a hydrogen ion implantation separation method has been proposed in which a single crystal silicon layer is formed over a supporting substrate to serve as a photoelectric conversion layer. Further, epitaxial growth, by a chemical vapor deposition method, of the single crystal silicon layer formed over the supporting substrate has also been proposed (for example, see Patent Document 1: Japanese Published Patent Application No. H10-93122).
Further, an atmospheric pressure plasma CVD method is known as a method to epitaxially grow a silicon layer (see Patent Document 2: Japanese Patent No. 3480448).